Tire width 120 or 110?

Need to replace the rear tire on my '79 KZ750. I believe OEM was a 110/90-18 but what was on there when I bought the bike is a 120/90-18.

I'm wondering what the pros/cons would be of going back to the OEM tire size.

Modern bikes seem to have much wider tires than what our old classics do. How does that effect the handling? Would I be better off with the tire size the bike was designed for? Was it just more fashionable to have skinny tires in the 70's or was rubber more expensive? Would the difference between a 110 and a 120 really be enough to matter?

steer quicker...small amount really in difference

120 is a good tire...do you have the chain guard...it hits first imo..

120

Not a big deal, but a 110 was never stock size on a KZ. Back then we had 4.00-18 and those kind of number system. Yes 110 is "replacement" for a 4.00, but not the same because a 110/90 is a 90 aspect ratio tire where 4.00 is 100% aspect meaning a taller tire. Some companies even list the tires as the same. That's like saying a 1/4" american bolt is "same" as a 6mm metric. Close but no. There are correct size tires available so why do wrong tires 110 or 120? Metzler Lazers and Avon Roadrunners are good tires still come in correct sizing as well as less pricy tire brands.
All brands are a little different, but just to sum up what to expect. A 110/90-18 is the correct basic width but low profile and looks wrong. A 120/90-18 is too wide for the stock 2.15" wheel so tire profile is compromised, but the diameter is close to standard 4.00-18. Sure guys run 130, 150 all kind of wide tires on a 2.15" rim and think they are doing the cool thing, but it's not right in looks or function.
Tire rant over

Thanks for the info!

The bike currently has a 120/90-18 on it. What I'm wondering is whether or not I'd notice any performance difference going with a 110/90-18. I guess the same question would apply to a 4.0-18 (if I can find one I can afford).

I don't really have any problem with the way the 120/90-18 looks, but I suppose that's a matter of taste.

Topper wrote:

...bike currently has a 120/90-18 on it. What I'm wondering is whether or not I'd notice any performance difference going with a 110/90-18. I guess the same question would apply to a 4.0-18 (if I can find one I can afford).
I don't really have any problem with the way the 120/90-18 looks, but I suppose that's a matter of taste.

whether or not I'd notice any performance difference --- likely not.

Had a 130 on the Z1, looked too fat for the rim (like a belly pooching over a tight belt) :pinch:, so went to more stockish 110, which now looks a little skimpy :S .

Next change will be to 120.

Good Fortune!

i know of a really sneaky basically stock looking 78 1000a2 with factory paint and a header running a very healthy 1385 motor lock up clutch and 36s that runs a 120 metzeler back tire! :laugh:

Topper wrote:

Need to replace the rear tire on my '79 KZ750. I believe OEM was a 110/90-18 but what was on there when I bought the bike is a 120/90-18.

No, actually the stock rear was 4.00 - 18 and front was 3.25 - 19. The closest equivalent to a 4.00-18 is 110-90-18. However, it is not identical and the one I have is a shade smaller than stock.

A 120-90-18 will fit the swing arm and is roughly like a 4.75 - 18 (which I also ran at one time). The only down side to running a fatter rear is that you will wear the center strip of the tread sooner because the tire is a bit wide for the rim, so it "pulls" the edges of the tread patch up a bit. The 110-90-18 that I currently have is wearing perfectly, it just looks a shade small for the wheel.

Topper wrote:

Thanks for the info!

The bike currently has a 120/90-18 on it. What I'm wondering is whether or not I'd notice any performance difference going with a 110/90-18. I guess the same question would apply to a 4.0-18 (if I can find one I can afford).

For a while, I was getting tires from JC Whitney that actually had both metric and English sizes on the sidewall. Don't know if they were fudging, but the sizes are not identical. I don't know anybody selling true Engish size tires anymore (?)

If you drop from 120 to 110 you might notice your cruising RPM go up a shade because of smaller diameter.

110 on Lesters

[Click on image to enlarge view.]

bountyhunter wrote:

The only down side to running a fatter rear is that you will wear the center strip of the tread sooner

Well that's pretty much what's happened with the old tire. The center strip of tread is almost totally worn down.

Look at dennis kirk for the 4.00-18 and 3.25-19 if you want to go stock size. I've been running these Avon Roadrunners on my 1977 KZ650 for a few thousand miles and they are working out real fine, so I ordered another set to replace the 20yr old rubber on the other KZ650 I took a spill on last Saturday. Ordered them tues and already have them. Dennis Kirk had the best price with shipping. They have a few different brands still offered in 4.00 type sizing.

I have also heard many good things about the Avon's, including that the sizing is still correct for the older bikes. I'm surprised Tri hasn't piped up as he has sold his soul to Avon, I believe.